The present invention relates generally to control tuning in a manufacturing process. Specifically, the invention relates to the optimization of a process control device when measuring and controlling the physical properties of articles moving through a manufacturing process.
Industrially manufactured products, such as, for example, tobacco cigarettes, extruded plastics, or steel billets, vary widely in physical makeup. The speed of movement of these materials as they are being processed may also vary widely. Some processes may move a work piece at a very slow rate of speed, while others move a work piece at speeds upwards of 40 mph.
However, in each of these manufacturing processes an ongoing need exists to measure and control the physical properties of the articles being made. Several devices have been designed and utilized to provide consistency in the physical properties of a line of manufactured articles. These devices may monitor such properties as length, weight, texture, and so forth. The results of this monitoring may be used to adjust parameters of the production process.
Differences between a target measurement for a given physical property of a manufactured article and its actual measurement can lead to excessive scrap, wasted machine time or lost orders, as the end product may be rejected by a demanding customer. Known systems have failed to provide for adequate control over the properties of such manufactured articles, due at least in part to their attempts to monitor and adjust the manufacturing process based on the short term standard deviation of a measured property, for example, the weight deviation from a target weight. Therefore, a need exists for a system and method of ensuring that the finished properties of a manufactured article come as close as possible to corresponding target values.
The present invention satisfies this need. The system and method of the present invention examines long term standard deviation that occurs during the manufacturing process and makes adjustments accordingly. For example, in one particular embodiment of the present invention, a system and method for measuring and controlling the weight of a flow of material is contemplated. More generally, however, the present invention provides a system and method for “fine tuning” a manufacturing process to more accurately produce a given article.
According to the system and method of the present invention, measurements of a particular physical property of a flow of material are captured at synchronized intervals over the length of the flow, and proportional and integral calculations are thereafter performed on these measurements. A measurement device is preferably used to capture the desired measurements of the physical property of the material at proper intervals. A material adjusting device, in communication with a controller and control loop, is preferably used to alter one or more process parameters so as to sufficiently maintain the particular physical property of the material at a target setpoint. Based on the results of the proportional and integral calculations, the flow of material may be acted upon to compensate for the duration of any deviation in the measurements from a target set point, and further deviations may be anticipated such that the total amount of deviation is reduced.
The system and method of the present invention preferably employs a proportional, integral, derivative (PID) algorithm and control loop to make the calculations and adjustments necessary to fine tune the action of a particular material adjusting device. In certain embodiments of the present invention, preferably only the proportional and integral portions of the PID algorithm are utilized, although the derivative portion may also be used if desired.
Essentially, the proportional portion of the control loop measures the error or deviation between a setpoint physical property value and a measured physical property value of the flow of material. Under proportional control, an attempt is made to adjust the output of a material adjusting device so that any error between the setpoint physical property value and a measured physical property value is removed. This is accomplished by the amount of change that will occur in the output of the material adjusting device as a result of a change to a corresponding input thereof. With error and gain known, the bias of the PID controller may then be adjusted (or the controller “reset”) in order to move the output of the material adjusting device as necessary to cause the physical property of interest to reach the setpoint value. The integral portion of the PID control loop is then used to continually and more accurately adjust the bias of the controller. Without the integral portion of the control loop, bias adjustments to the controller would have to be accomplished manually. To more accurately tune the device, the integral portion of the PID control loop may effect an automatic bias adjustment (automatic reset) whenever an error between the setpoint value and measured value are detected.
To accomplish fine tuning, small adjustments (bumps) are periodically and manually made to the input of the material adjusting device. In response to the input change, the output of the material adjusting device will cause movement thereof for a specified amount of time. Individual physical property measurements are taken, preferably from a time before each bump to the input is initiated, through a time after the output has fully responded to the change in the input. Physical property measurements are collected for each of the bumps, and the collected data is fed into an optimization program. Based on the collected data it receives, the optimization software can then generate tuned control parameters for use by the PID control loop. Therefore, by utilizing the tuned control parameters, the system and method of the present invention allows the mean value of the physical property to be more quickly and more accurately adjusted and controlled than is possible with known systems and methods.